Currently there is a revival in interest in 3D television, this is related to recent breakthroughs in display technology which allow good reproduction of 3D video for multiple viewers. One of these is the autostereoscopic 3D lenticular display, but there are also other types of displays, such as the autostereoscopic barrier based displays and the time-multiplexed stereo displays based on rear projection technology. Typically these types of display use one of two basic video formats as an input to create a 3D impression to the viewer. The stereo based displays use time sequential interleaving and glasses to display two separate views, one for each eye, and therefore expect stereo video as an input. Examples of these are the time-multiplexed rear projection based stereo displays and this type of system is also used in 3D cinema. The main alternative to this are multi-view autostereoscopic displays, which require no glasses and often use an input format known as image+depth as an input to generate a 3D impression. More information on 3D display technology can be found in chapter 13 of “3D video communication—Algorithms, concepts and real time systems in human centered communication” by Oliver Shreer et al. (Wiley 2005).
The stereo video format is straightforward as it provides two images, one for each eye. Typically these two images are interleaved, either spatially or in a time-sequential manner and then fed into the display. The alternative format, called image+depth, is different in that it is a combination of a 2D image with a so called “depth”, or disparity map. This is typically a gray scale image, whereby the gray scale value of a pixel indicates the amount of disparity (or depth in case of a depth map) for the corresponding pixel in the associated 2D image. The display when rendering the 3D image uses the disparity or depth map to calculate the additional views taking the 2D image as input. This may be done in a variety of ways, in the simplest form it is a matter of shifting pixels to the left or right dependent on the disparity value associated to those pixels. The paper entitled “Depth image based rendering, compression and transmission for a new approach on 3D TV” by Christoph Fen gives an overview of the technology.
A problem with the autostereoscopic and (time sequential based) stereo 3D displays is what is referred to as the accommodation-convergence mismatch. This is a problem in which a viewer's eyes converge on the virtual location of objects being displayed, whilst at the same time the eyes accommodate (to see the image sharp) on the surface of the display itself. This mismatch may cause headaches and other symptoms associated with motion sickness. In addition any geometrical (especially any vertical parallax), as well as electrical (brightness, contrast etc.) differences between the views for the left and right eyes can additionally cause visual fatigue. However, if the amount of disparity is kept small, that is smaller than one degree, then these problems are less severe and users can view the content without any significant problems. See “Two factors in visual fatigue caused by stereoscopic HDTV images”, Sumio Yano et al. Displays 2004 pages 141 to 150 Elsevier, for more detail.
Similar to the above problems are that a problem can occur when the rendering device is displaying text such as subtitles or closed captioning. If the text is not properly positioned on the screen, which depends on the type and settings of the display, the text may appear blurred, for example owing to crosstalk between the left and the right eye views, and the viewer may feel tired. It is also the case that the blurring can impact on the readability of the text. According to E. Legge (see “Psychophysics of Reading: I. Normal Vision”, Gordon E. Legge et. al. Vision Research, Vol 25, No. 2, pages 239 to 252, 1985) reading is impaired if the bandwidth of the text is below two cycles per character. Blurring is a problem with auto stereoscopic displays, as typically resolution is sacrificed to generate multiple views, and for stereo displays in general there is a problem with a suboptimal separation between the two views, which can add to blurring of the image. Furthermore according to Yano (referenced above), depth motion increases visual fatigue.
Another problem that is foreseen, is that viewers can adjust the amount of disparity and relative position of the depth plane in a 3D television (through for example some buttons on their remote control). These adjustments mean that the text may become blurred, as it moves away from a depth neutral position, or increases in “depth” such that it causes visual fatigue.
United States of America Patent Application Publication US 2005/0140676 discloses a method for displaying multi-level text data in three-dimensional map. In the system described in this Publication, a three-dimensional map is displayed on a screen, and text data with different levels of density are displayed according to distances from a view point of the displayed three-dimensional map to nodes where the text data will be displayed, thereby improving readability of the text data. Further, it is possible to display the text data by locally adjusting the density of the text data on the screen. The three-dimensional map is displayed on the screen of a display panel by converting map data with two-dimensional coordinates into those with three-dimensional coordinates by means of a perspective projection method. Text data to be displayed together with the three-dimensional map are converted into those in a three-dimensional coordinate system with an origin defined by the view point of the three-dimensional map. The converted text data are projected on a two-dimensional plane to be converted into those with screen coordinates. Then, distances from the view point of the displayed three-dimensional map to the nodes where the text data will be displayed are classified. The classified distances are determined for the converted text data with the screen coordinates. Text data of levels corresponding to the determined distances are displayed on the screen of the display panel on which the three-dimensional map is displayed.
While in the context of the representation of a three-dimensional map on a two-dimensional display device, the handling of the text data according to this publication is positioned and scaled in a meaningful manner for the user, it does not address any of the issues identified above, relating to the display of text in a three-dimensional display device.